Fixes grack authentification under relative_url_root

Ref: https://github.com/gitlabhq/gitlabhq/commit/e6159b8725f99af78f446f8d33fa0e52b7780430
Ref: https://github.com/gitlabhq/gitlabhq/pull/3204
Ref: https://github.com/gitlabhq/gitlabhq/issues/1228

Add Rails' variable in application.rb to support relative url

This variable is used by assets compilation and other modules.

Note that user needs to change application.rb too

Restrict session cookie to the relative path if set.

Ref: https://github.com/gitlabhq/gitlabhq/commit/2c2f1e31856a4decdae469974f5bea8245316f7e

Fix Update attachment_uploader.rb bug with relative URL

See: https://github.com/gitlabhq/gitlabhq/commit/161afda3fa4fca58f396e9c3acbd72bc14490ace

Fix Wall relative bug with attachement files (javascript)
Make flash messages fixed and sticked to bottom
Update flash message timeout to 5 seconds
Merge pull request #4797 from mbajur/flashes-revived

Make flash messages fixed and sticked to bottom
Merge pull request #4670 from anezi/relative-url-patch

More fixes for supporting Relative URL
its time for beta
Make repo graph work for large repos on slower machines
Configure the relative path only one time.
Fix confusing situation where install guide told you to install init.d script for puma.
Relative root needs to be changed in three places.
Merge branch 'init-script-should-restart-unicorn' of /home/git/repositories/gitlab/gitlabhq
Better message for migrating global projects
Its time for 6.0.0rc1
Rename wiki repo if project repo was removed
Dashboard event right margin + increased padding
Fix event image preview
Render small avatars for inline events
Bold names for public projects
Time for 6.0.0
Fix bug in installation doc with non-install unicorn